1. Field of the Invention
Embodiments of the present invention relate to a charged particle beam writing apparatus and a charged particle beam writing method. For example, they relate to a method for obtaining a dose of electron beam radiation in the electron beam writing.
2. Related Art
The microlithography technique which advances microminiaturization of semiconductor devices is extremely important as being a unique process whereby patterns are formed in the semiconductor manufacturing. In recent years, with high integration of LSI, the line width (critical dimension) required for semiconductor device circuits is decreasing year by year. In order to form a desired circuit pattern on semiconductor devices, a master or “original” pattern (also called a mask or a reticle) of high precision is needed. Thus, the electron beam writing technique, which intrinsically has excellent resolution, is used for producing such a highly precise master pattern.
FIG. 15 is a schematic diagram for explaining operations of a variable-shaped electron beam (EB) writing apparatus. As shown in the figure, the variable-shaped electron beam writing apparatus operates as follows: A first aperture plate 410 has a quadrangular, such as a rectangular, opening 411 for shaping an electron beam 330. A second aperture plate 420 has a variable-shape opening 421 for shaping the electron beam 330 that has passed through the opening 411 into a desired rectangular shape. The electron beam 330 emitted from a charged particle source 430 and having passed through the opening 411 is deflected by a deflector to pass through a part of the variable-shape opening 421 and thereby to irradiate a target workpiece or “sample” 340 mounted on a stage which continuously moves in one predetermined direction (e.g. X direction) during the writing. In other words, a rectangular shape capable of passing through both the opening 411 and the variable-shape opening 421 is used for pattern writing in the writing region of the target workpiece 340 on the stage. This method of forming a given shape by letting beams pass through both the opening 411 of the first aperture plate 410 and the variable-shape opening 421 of the second aperture plate 420 is referred to as a variable shaped beam (VSB) method.
In the electron beam writing described above, the dose of each shot is set such that a beam dose at the figure edge of a figure configured by connecting a plurality of shots is to be a threshold value of a dose required for resist pattern resolution. Usually, it is set such that about a half of a shot dose at the figure edge reaches the threshold value. For calculating a dose, one dose equation is used regardless of position of the irradiation. Therefore, when writing a figure configured by connecting a plurality of shots, the dose is set in each shot such that about a half of the dose reaches a threshold value irrespective of whether it is at the figure edge or not.
In the meantime, with recent miniaturization of patterns, writing time of the writing apparatus becomes long, and thus, shortening of the time is required. However, in order to write a pattern in accordance with sizes, it is necessary to make a calculated dose incident on a resist, and thus, the conventional method has a limit of reducing the writing time.
Here, in relation to a dose equation used in the electron beam writing, there is disclosed a method of calculating a dose, in order to correct a phenomenon called a proximity effect etc., by changing values, such as a base dose Dbase used for calculating a dose and a proximity effect correction coefficient η used for correcting a proximity effect, depending on positions. (Refer to e.g., Japanese Patent Application Laid-open (JP-A) No. 2007-150243). However, even in such a method, the dose equation used is the same one wherein variables are adjusted.
As described above, conventionally, each shot dose is calculated by using one dose equation which is input to the writing apparatus. When performing irradiation based on an incident dose calculated by the conventional dose equation, the total dose at each of all the regions except for a figure edge and a place on which nothing is written is larger than a threshold value of the resist. In order for the total dose at the figure edge to be a threshold value of the resist, the total dose in the vicinity of the figure edge needs to be larger than the threshold value of the resist. However, the total dose of a region sufficiently distant from the figure edge may be about the threshold value. This subject has not been taken into consideration in the conventional method. Therefore, for example, in the case of writing a figure configured by connecting a plurality of shots, when an incident dose of a region distant from the figure edge, sufficiently farther than the radius of forward scattering of the beam, is calculated by the conventional method, the dose of the region is larger than the threshold value of the resist. That is, when a dose is large, the irradiation time becomes long in accordance with the dose. Thus, as has been mentioned, an excessive dose exists depending on a figure or its irradiation position, and accordingly, there is a problem of taking a writing time longer than needed because of such excessive dose.